The present invention relates generally to optics, and more particularly, to combining I-Q and/or PolMux optical receiver to enable single detector.
Dual-polarization QPSK/xPSK/xQAM (DP-QPSK/xPSK/xQAM) modulation with coherent receiver and digital signal processing is becoming the dominant technology for long-haul transmission, because of the high spectral-efficiency, the tolerance of polarization-mode dispersion, the ability of full-range chromatic dispersion compensation, and the resulting excellent performance. FIG. 1 is the block diagram for one polarization from prior art using this approach. Received signal E(t) has both in-phase and quadrature elements. Both E(t) and locally generated optical signal ELO are first split into two copies using 3-dB coupler. One copy of signal from local oscillator is phase shifted by 90-degree. Then one copy of E(t) is coupled with ELO, and the other copy of E(t) is coupled with the 90-degree shifted ELO. Both are outputted to a balanced detector and converted to electrical signals, namely in-phase (I) and quadrature (Q) signals respectively.
One obvious drawback for receiver in FIG. 1 is the requirement of 4 photo-detectors (PD, or balanced PD pairs) to receive both polarizations, with each connected to one high-speed ADC. This makes the receiver complicated and more power demanding. As for high-speed ADCs, technology innovations enable even higher rate with wider bandwidth. The extra sampling capability will be wasted if optical signal bandwidth is below (for example, half of) the maximum ADC capability. The present invention provides an approach to utilize the excessive bandwidth/sampling-rate of the high-speed ADCs to reduce the required number of photo-detectors and ADCs.
Accordingly, there is a need for signal detection that overcomes shortcomings of the prior efforts.